Growth of carbon nanotubules on Fe-loading zeolites and investigation of catalytic active center

Direct synthesis of carbon nanofibers from South African coal fly ash

Carbon nanofibers (CNFs), cylindrical nanostructures containing graphene, were synthesized directly from South African fly ash (a waste product formed during the combustion of coal). The CNFs (as well as other carbonaceous materials like carbon nanotubes (CNTs)) were produced by the catalytic chemical vapour deposition method (CCVD) in the presence of acetylene gas at temperatures ranging from 400°C to 700°C. The fly ash and its carbonaceous products were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), laser Raman spectroscopy and Brunauer-Emmett-Teller (BET) surface area measurements. It was observed that as-received fly ash was capable of producing CNFs in high yield by CCVD, starting at a relatively low temperature of 400°C. Laser Raman spectra and TGA thermograms showed that the carbonaceous products which formed were mostly disordered. Small bundles of CNTs and CNFs observed by TEM and energy-dispersive spectroscopy (EDS) showed that the catalyst most likely responsible for CNF formation was iron in the form of cementite; X-ray diffraction (XRD) and Mössbauer spectroscopy confirmed these findings.

The role of alpha-iron and cementite phases in the growing mechanism of carbon nanotubes: a 57Fe Mössbauer spectroscopy study

57Fe Mössbauer spectroscopy was used to study the reduction behavior at temperatures as high as 1073 K of an iron/silica catalyst, and also the carbonaceous materials isolated after acetylene decomposition over this catalyst at several temperatures (873-1073 K). The products were previously characterized by transmission electron microscopy and it was clearly proven that the concentration of carbon nanotubes increased when reaction reached highest temperatures. This was related with an increment in cementite concentration (generated from initial alpha-iron and the progressive reduction of the remnant Fe+2 caused by acetylene decomposition) as detected by 57Fe Mössbauer. These results undoubtedly revealed the role of alpha-iron as active center for acetylene decomposition and cementite as main carbide intermediate species in the catalytic growth of CNTs.